[Insight-users] MultiResMIResgistration - scaling

[Luis Ibanez]

Hi Christos,

Rigid transform do not incorporate scaling.
If you need scaling you should use the
"SimilarityTransform" or the "AffineTransform".

BUT, Note that you *DONT* need scaling when
registering multi-modality images of the same
patient unless:


A) The patient has been attacked by a tribe
    of head-shrinkers after getting out of one
    scanner and before going into the other
    scanner

B) The patient suffers from hydrocefalia.


ITK performs registration in *Physical Coordinates*,
not in pixel coordinates. The number of pixels of
your image is irrelevant as long as the pixel spacing
is set correctly.

What you *must* verify is that the physical extent
covered by both the MRI and the PET images correspond
correctly to the normal size of a human head in millimeters.

For example, it is likely that your MRI image has dimensions

   256 x 256 x 55 Pixels

with pixel spacing of about:

   0.5mm x 0.5mm x 3.0 mm

which results in a physical extent of


   128.0mm   x  128.0mm  x 165.0mm

The PET image may be something like, dimensions

    128 x 128 x 31 pixels

with pixel spacing

     1.0 mm  x  1.0 mm  x  3.0 mm

which results in a physical extent of


     128.0mm  x 128.0mm  x 93.0mm


Therefore they cover approximately the same region
of physical space.

For succeeding in image registration it is fundamental
for you to first verify that your images have correct
pixel spacing and image origin expressed in millimeters.



The choice of the image that is going to be the
moving/fixed image is arbitrary. In the case of MRI
versus PET you probably want to use the MRI image as
the fixed image and the PET image as the moving one.
In that way, at the end of the registration process
the PET image will be resampled in the coordinate
system of the MRI image.


--

BSplines will be able to manage scaling. BUT you
probably DONT NEED scaling. Make sure that you are
comfortable with rigid registration before you move
in the more complex registration processes.




Regards,



    Luis



-----------------------------
Christos Panagiotou wrote:

> hi all
> some questions concerning multiResMiRegistration
> 
> i am trying to use the MultiResMIRegistration application
> to register PET and MRI volumes. I would like to ask if the rigid 
> transformation
> incorporates scaling in the registration procedure or it implements 
> translation and rotation but not scaling
> 
> i tried to register a PET volume (128x128x31) against an MRI volume 
> (256x256x55). The application allies them
> however it does not scale the bigger volume to to "fit" the smaller one. 
> It somehow "crops" the MRI (larger) volume
> to fit the 128x128x31 of the PET. Is there any option to incorporate 
> this? i ve seen definitions of rigid registration that incorporate 
> scaling and some that they dont...
> 
> also, if volumes are of different dimensions, should the moving volume 
> always represent the smaller volume (from the two)?
> 
> finaly, do you know if the BSplineDeformableTransform incorporates 
> scaling (as it is non-rigid i assume it must do)
> 
> thank you all
> any help will be appreciated
> christos
> 
> 
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